In vivo molecular imaging makes it possible to measure local biochemical and pharmacological parameters non-invasively in intact animals and humans. Molecular imaging includes nuclear techniques, magnetic resonance imaging (MRI), and optical techniques. Among nuclear techniques, positron emission tomography (PET) is the most sensitive technique and the only one to offer quantitative measurements.
The quantification of measurements with PET for the study of a drug requires the intravenous injection of a radioactive tracer adapted to the study of that drug. An image of the tracer distribution is then reconstructed. The time activity curves (TAC), i.e. the variations of the tracer concentration within a volume element, are also highly spatially correlated due not only to the reconstruction method, but also to the existence of physiological regions that respond to the tracer identically, regions that can be called pharmaco-organs.
The post-processing of the PET image generally requires three steps to access the pharmacological parameter. First, regions of interest (ROI) defining the organs, more precisely the pharmacokinetic organs, must be defined either on the PET image or on a high-resolution image matched point-to-point with the PET image. Next, the TACs of the pharmaco-organs must be extracted and possibly corrected to offset the limited spatial resolution of the PET. Lastly, a physiological model can be defined, based on the TACs and the tracer concentration in the plasma, making it possible to calculate pharmacological parameters that are interesting for the biologist. A precise definition of the ROIs is necessary so as to extract the relevant TACs.
However, the quantification of the TACs is hindered by the limited resolution of the PET system, resulting in what is called the partial volume effect (PVE).
A known geometric transfer matrix (GTM) method makes it possible to correct the PVE effectively. This method requires the data from the PET image on one hand, and on the other hand, space domains defining the functional organs, regions of interest within which the average TACs of the functional organs will be calculated, and a resolution model of the PET imager.
However, this GTM method is very sensitive not only to the definition errors of said space domains and the image reconstruction artifacts, but also to the image smoothing effects due to periodic physiological movements, such as heartbeats or respiratory movements.